Electrostatically assisted ink transfer



Jan. 1970 K. B. MORRIS 3,489,082

ELECTROSTATICALLY ASSISTED INK TRANSFER Filed Nov. 20, 1967 2 Sheets-Sheet 1 Jan. 13, 1970 B MORRlS 3,489,082

ELECTROSTATICALLY ASSISTED INK TRANSFER Filed Nov. 20, 1967 2 Sheets-Sheet 2 I I W e/m 70/2 m r/m/a 5 05m A m 5 5 P Z Z United States Patent 3,489,082 ELECTROSTATICALLY ASSISTED INK TRANSFER Kenneth B. Morris, London, England, assignor to Crosfield Electronics Limited, London, England, a British company Filed Nov. 20, 1967, Ser. No. 684,330 Claims priority, application Great Britain, Nov. 24, 1966, 52,703/ 66; May 23, 1967, 23,943/67 Int. Cl. B41f 9/06 US. Cl. 101-153 13 Claims ABSTRACT OF THE DISCLOSURE In a printing operation, a web to be printed passes between a printing roller and a conductive cylindrical member of high electrical resistance and the transfer of ink from the printing roller to the web is electrostatically assisted. To reduce the risk of sparks, a generator for generating a voltage to create the electrostatic field includes voltage multiplying means enclosed within and rotating with a roller, itself rotating with the printing roller, and the voltage is applied from the output of the voltage multiplying means to the conductive cylindrical member without external leads.

Images printed by gra-vure printing sometimes appear speckled with white dots. This effect is known as speckle or dot skip and occurs when ink in some of the gravure cells on the printing cylinder is not transferred to the paper. The effect is particularly prevalent with low-grade paper the surface of which may contain small cavities, at which points the ink does not contact the paper and thus is not drawn out of the cells. The present invention is concerned with electrostatically assisting the transfer of ink in such cases and the invention can also be applied to the letter-press process.

Many years ago it was proposed to form the outer surface of the impression roller with conductive rubber and to apply a high voltage between a steel roll forming the core of the impression roller and the printing cylinder. A very high voltage is necessary and the electrostatic force then operates on the ink, which is of comparatively low viscosity, to deform the surface and thereby improve the chance of the ink making contact with the paper. However, although is using systems of this kind it has been established that an improved ink transfer results, such systems suffer from the disadvantage that they do not conform to the safety standards applicable to rotary printing machines. Their principal drawback has been the danger that an electrical spark resulting from the high voltages employed could ignite the explosive vapours which are present in a gravure printing environment. One example of such apparatus is shown in British patent specification No. 619,224. In these earlier proposals the impression roller is generally electrically insulated from the side of the press and thus from the printing cylinder and the voltage required for the electrostatic ink-transfer assistance is applied through a brush or sliding contact to a slip ring on the journal of the impression cylinder. The impression roller has a coating of rubber having a moderate electrical conductivity so that a voltage appears at the surface of the roller and creates a strong electrostatic field between the paper and the printing cylinder. The impression roller may be insulated from the side of the press by means of its bearing mountings. In such an arrangement, however, the brush and slip ring connection constitute an explosion hazard and in addition the electrical capacitance between the main journal of the impression roller and earth (including the capacitance across the insulated bearing mountings, the

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capacitance of the voltage feed line and all stray capacitances) would on many large printing presses be of such a value that the energy stored would constitute a further explosion hazard since an accidental short circuit across the capacitance could result in a spark with an energy level sufficient to cause ignition of the solvent vapours in the vicinity. These hazards have been such as to prevent the widespread acceptance of apparatus employing the principle of electrostatically assisted ink transfer.

According to the present invention, a voltage multiplying means connected to receive a low input voltage is mounted within and rotates with a roller which itself rotates with the printing roller, the voltage multiplying means being totally enclosed within the roller, and its output is connected without external leads to a cylindrical conductive layer of high resistance which, in operation, is in contact with the web, this layer being insulated from the journal about which it rotates. The voltage multiplying means may be in the impression roller itself, but it is generally more convenient to mount it in an additional roller. In this case the output of the voltage multiplying means is fed to the surface of this additional roller and is transferred to the impression roller by surface-to-surface contact. At least one of these surfaces is of a resilient material so that in operation it flattens slightly to give an increased area of contact. Then, in the unlikely event of the two rollers losing contact while the press is running, there is a progressive reduction of the area of contact before the final break and this results in an increase in resistance between the rollers which progressively reduces the current so that when contact between the roller is lost the current has a negligible value.

In the preferred arrangement, the voltage multiplier includes as its output section a capacitor-rectifier voltage multiplying circuit having a high output impedance. This has the advantage that if there is a short circuit a high current cannot flow owing to the limiting action of the high output impedance. This voltage multplying circuit may be preceded, for example, by a transistor oscillator utilising a step-up transformer.

To provide the low voltage for the voltage multiplying means, we prefer to use a generator which is housed within the roller containing the voltage multiplying means and which is so arranged that it generates voltage as a consequence of rotation of the roller. With this arrangement there are no external leads and moreover there can be no high voltage on the rollers if the press is stationary. This generator can also be used in a system for controlling the voltage applied across the web. This can be achieved by extending out the shaft of the part of the generator which is not fixed for rotation with the roller so that it passes through the roller casing and by providing variable breaking of the projecting part of this shaft. For maximum voltage the shaft is held stationary; to reduce the voltage the shaft is allowed to slip. This permits further safety features to be built into the apparatus; the brake can be lifted away from the shaft by a sensing device in the event of a short circuit and may also be lifted away from the shaft by an interlocking arrangement when the press is stationary. This latter feature prevents the generation of voltage by manual rotation of the rollers when the press is switched off.

It will be seen that the combination of a rotating voltage multiplying means having a high output impedance and a self-generating roller with an interlock to prevent voltage generation by manual rotation, provides a very high measure of safety.

In order that the invention may be better understood, some examples of apparatus embodying the invention will now be described with reference to the accompanying drawings, in which:

FIGURE 1 shows the general arrangement of the rollers in a first apparatus;

FIGURE 2 shows the construction of the roller containing the voltage-multiplying means;

FIGURE 3 is a block diagram of the voltage-multiplylng means;

FIGURE 4 is a circuit diagram of the apparatus shown in block form in FIGURE 3;

FIGURE 5 shows an alternative construction in which the voltage-generating apparatus is mounted on the end of the impression roller; and

FIGURE 6 illustrates an arrangement with an external generator.

As shown in FIGURE 1, a web 10 runs between the surface of the gravure printing cylinder 11 and a surface layer 12 of an impression roller. The layer 12 is of moderately conductive rubber, for example rubber loaded with graphite. The layer 12 is insulated from a steel core 13 of the impression roller by an intermediate cylindrical layer 14 of highly insulating material.

2 The conductive rubber layer 12 derives its voltage from a similar cylindrical layer 15 forming the outer surface of an additional roller. The layer 15 in turn receives voltage from an underlying metal layer 16 of foil or wire mesh which is separated from a steel casing 18 by an insulating layer 17.

The construction of the additional roller is shown in greater detail in FIGURE 2. The steel shell 18 encloses an electrical generator 20 the output of which goes to the voltage multiplying means 21. One side of the output of the voltage multiplying means is connected to the steel shell by way of a lead 22. The other side is connected through a lead 23 to a conductive pin 24 which extends through the steel shell 18 in an insulating bush 25 and through the insulating layer 17 to the metal foil or wire mesh layer 16. The additional roller thus serves as a generator roller. A light spring pressure is applied to the journals of the generator roller to ensure good contact with the outer layer of the impression roller.

The generator may be of any kind such that the rotor and stator can be given relative rotation as a consequence of the rotation of the impression roller. In the apparatus which is being described, what would normally be the stator Winding rotates with the impression roller and what would normally be the rotor winding is connected to a shaft 26 which extends out of the end of the additional roller to permit the relative rotation to be achieved by the clamping or braking of this shaft. In the example shown, a small wheel 27 is mounted on the end of the shaft. A brake shoe 28 is mounted on bellows 29 the extension of which is controlled by air pressure in an air line 30. Thus, a variable degree of braking can be applied to the steel 27. If excessive current passes through the generator windings then the motor eiTect will cause adequate torque to be generated to overcome the braking action on wheel 27. This reduces the relative speed of the rotating and nonrotating parts of the generator and thus reduces its output. The air pressure in the air line 30 is also governed by an interlocking arrangement which automatically removes the air pressure as soon as the press is switched off. Then if a oller is manually rotated the shaft 26 tends to rotate with it and thereby reduces any voltage generated at the surface of the roller.

Finally, the air line 30 is connected to a regulator which, by varying the pressure in the line and thereby the degree of slip permitted for the shaft 26, controls the voltage output. This permits an operator to adjust the voltage generated within the roller while the press is running without an external electrical control. As an example, the opeartor may slowly increase the air pressure from zero when the press is started up until the required degree of improvement on the web had been obtained.

The arra gem nt descr bed for pe m t g e shaf 26 to slip is particularly advantageous because of the feedback elfect of the generator. Increased pressure in the line 30 reduces the slip of the shaft 26, increases the relative speed of the rotating and non-rotating parts of the generator, and increases the voltage output of the latter with the result that the current increases and reacts on the shaft 26 in opposition to the increased force applied to the wheel 27. This is a kind of negative feedback and renders the arrangement very stable.

As shown in FIGURE 2, the ends of the steel shell of the additional roller are sealed by screwed journal ends 31 and 32 to form an explosion-proof chamber. As the shaft 26 passes through the journal end 32, the length of the journal through which the shaft passes and the clearance between the shaft and the journal must be such as to comply with safety standards. The requirements for this are already known in connection with the construction of fiameproof electric motors.

The block diagram of FIGURE 3 ShOWs that the output of the alternator 20 is first applied to a rectifier 35 in the voltage multiplying means 21. The rectified voltage is applied to a voltage limiting circuit 36 and thence to an oscillator 37 providing an AC. signal at a frequency of about 10 kc./s. This signal is applied to a condenserrectifier voltage-multiplying circuit 38.

In the more detailed diagram of FIGURE 4, for convenience the alternator 20 is shown separate from the roller 18 in which it is housed. The rectifier 35 is a conventional full-wave bridge rectifier and the voltage limiting circuit 36 includes a transistor 45 having its collectoremitter path in the current supply line for the oscillator. The base of this transistor is connected through the emitter-base path of a further transistor 46 to the collector of a transistor 47. The base of this is controlled in accordance with the required output voltage, as will be described later.

The oscillator circuit 37 is of conventional design and includes two transistors 48 and 49 in an inverter circuit and a transformer 50 providing an increased voltage in its secondary winding. This increased voltage is applied across a voltage-multiplying rectifier-capacitor network of conventional design, the multiplied voltage being obtained on the output conductors 54 and 55. Two resistors 56 and 57 are connected between these conductors, the resistor 57 being adjustable and set to limit the output of the voltage limiting circuit when the multiplier output has attained a desired high level thus limiting the multiplier output to that desired level. Thus limiting action is achieved by connecting the junction of the resistors 56 and 57 to the base of the control transistor 47 by way of conductor 58.- Conductor 59 connects the positive side of the voltage multiplier to the emitter connection of the transistor 47.

The output of the voltage multiplier is applied to a polarity switch 60. One pole is connected through the switch to the metal shell 18 of the generator roller and the other side is connected to the metal coating 16, which may be in the form of a layer of foil or may be a mesh. The polarity switch is provided because we have discovered that for some types of ink the transfer is improved if the voltage at the back of the web is of negative polarity, while other inks require a positive voltage.

Although we have shown the generator roller as an additional roller, it is possible to mount the generating components within the impression roller itself.

FIGURE 5 shows an arrangement in which the voltage-generating components are mounted on the end of the impression roller in a hollow ring 65. This ring rotates with the impression roller and contains pole pieces and coils in which a voltage is generated by virtue of their relative rotation with respect to a ring 66 of stationary permanent magnets. The ring 65 also contains the voltage-multiplying means. One pole of the output of the volt-. age-multiplying means is electrically connected to the, Shaft 67 and the other pole is electrically connected to a metallic layer 16a between the conductive rubber layer 12 and the insulating layer 14 of the impression roller. The stationary magnets may, of course, be arranged radially outwards from the coils instead of at the side of them and the ring of magnets may be split diametrically to facilitate easy mounting on an existing press.

FIGURE 6 shows a further arrangement in which the generator 70 is external to the impression cylinder but is still driven from the latter by a small driving wheel 71 which, when the arrangement is in use, contacts the surface of the impression cylinder. The generator 70 provides a low voltage output and this is conveyed to a hollow ring 65 on the end of the impression cylinder by means of a brush 72 and slip ring 73. The multiplying means 21 can be fitted inside the hollow ring or inside the roller and, as in the case of FIGURE 5, the high voltage side of the circuit 21 is connected to a metallic layer under the conductive rubber layer 12 of the impression cylinder.

As an alternative to a rotary generator for providing the low voltage input to the voltage multiplying means, an electrical battery can be used and this can be arranged within the additional roller or can be external to the roller and connected through a brush and slip ring. An external low energy low voltage supply (for example, 12 volts) can also be provided by a transformer or transformer and associated rectifying means, the output being connected to the voltage multiplying means through a brush and slip ring.

If a rotary generator is used, it is not essential to bring the shaft of its non-rotating part through the end of the generator cylinder. As an alternative, a weight can be eccentrically mounted on the shaft within the cylinder in order to prevent or reduce rotation of the shaft. In this case, the mass of this weight is so chosen that a degree of energy regulation is obtained so that if, for example, a direct short circuit to the generator terminal results in a demand for excessive current from the generator, this current produces a motor effect permitting the normally non-rotating part of the generator to rotate taking with it the mass. It can also be arranged that rotation of the eccentric Weight, when such a short circuit exists, physically disconnects the generator roller from the impression roller. As an example, the centrifugal force due to the rotation of the eccentric weight can be arranged to overcome a spring force to move the whole generator cylinder sideways to provide a clearance betwen the generator roller and the impression roller. This clearance can be increased by arranging for the lateral movement to cause an upward movement of the generator roller, for example by the use of a cam arrangement.

If a battery is used, a centrifugal switch within the roller can be arranged to cut off the battery voltage below a certain rotational speed. This switch is preferably arranged to precede the oscillator and voltage multiplying circuits.

Yet a further alternative is to use an A.C. output from the voltage multiplying unit in order to avoid the need for a polarity switch. With an A.C. output, because the voltage at the back of the web is positive and negative in alternate half-cycles, the maximum ink transfer takes place at some point in the cycle for all types of ink, provided that the voltage amplitude is large enough. The frequency of the A.C. voltage applied should then be related to the maximum speed of the web. As an example, for a web traveling at 2,000 aft/minute, a frequency of 4 kc./s. would give a complete cycle, including positive and negative halves, during the passage of about ,4 inch of paper. Since the circumferential width of the nip of the rollers would be more than inch, optimum transfer would be obtained without the streaking effect which would take place at low frequencies. The voltage multiplying means within the roller may be an A.C. transformer.

It will be understood that where brushes and slip rings are used, the voltage led in in this way is severely limited,

for example up to 12 volts at one ampere, and that the external supply should be of a low-energy kind. In addition, it will be understood that the slip ring and brush will be totally enclosed.

Although in letterpress applications there is no fire hazard, operator safety still has to be considered. In such applications, however, the generators need not be arranged in explosion-proof chambers.

I claim:

1. Printing apparatus comprising:

a printing roller;

a journal;

a cylindrical member mounted on said journal and formed of electrically resistive material, said member being positioned with respect to said roller so as to permit a web to be printed to pass therebetween during operation of the apparatus;

a voltage generator connected to supply voltage to said member to produce an electrostatic field between the web and said printing roller for aiding transfer of ink from said printing roller to the web, said voltage generator including voltage multiplying means and connections for applying an input voltage to said voltage multiplying means;

a further roller rotating with said printing roller, said voltage multiplying means being within and rotatable with said further roller; and

means excluding external leads for applying the output of said voltage multiplying means to said cylindrical member.

2. Printing apparatus in accordance with claim 1 further including a rotary generator connected to apply its output voltage to the voltage multiplying means, said rotary generator being housed within the roller containing the voltage multiplying means and being mounted so that its rotatable parts are driven by rotation of the roller itself, whereby no voltage is generated in the absence of rotation of said roller.

3. Printing apparatus in accordance with claim 2, including a shaft connected to the non-rotating parts of said rotary generator, the shaft extending outside said roller within which said generator and voltage-multiplying means are housed, the apparatus further including means outside said roller for preventing rotation of the shaft.

4. Printing apparatus in accordance with claim 3, in which said means for preventing rotation of the shaft comprise an adjustable braking device which can be adjusted to permit slipping of said shaft when it is desired to reduce the voltage output of said generator.

5. Printing apparatus in accordance with claim 4, in which said adjustable braking device is controlled by fluid pressure and in which the apparatus further includes a regulator for adjusting the voltage output of said generator.

6. Printing apparatus in accordance with claim 4, including interlocking means controlling said braking device so as topermit free rotation of the shaft when the press is stationary, thereby reducing voltage generated by manual rotation of the roller.

7. Printing apparatus in accordance with claim 1 including an impression roller, the outer layer of which is formed by said conductive cylindrical member of electrically resistive material, said impression roller constituting said further roller within which said voltage multiplying means is mounted.

8. Printing apparatus in accordance with claim 1, including an impression roller of which the outer layer is formed by said conductive cylindrical member of high resistance, said further roller which houses said voltagemultiplying means having an outer surface in the form of a conductive cylindrical layer of high resistance, and in which the output voltage of said voltage-multiplying means is applied to said outer surface of said further roller and thence to said outer layer of said impression roller.

9. Printing apparatus in accordance with claim 1, including a battery housed within said roller containing said voltage multiplying means and supplying the latter with its input voltage.

10. Printing apparatus in accordance with claim 1, in which the voltage-multiplying means consists of a transformer.

11. Printing apparatus inaccordance with claim 1, in which an external low energy and low voltage power supply feeds the voltage-multiplying means within said further roller through brushes and slip rings.

12. Printing apparatus in accordance with claim 1, in

which said voltage multiplying means includes an output 15 References Cited UNITED STATES PATENTS 2,408,144 9/1946 Huebner.

2,520,504 8/1950 Hooper 101426 2,558,900 7/1951 Hooper 101219 2,558,901 7/1951 Hooper 101426 2,590,321 3/1952 Huebner 1011 14 2,691,343 10/1954 Huebner 101170 XR 3,295,441 1/1967 Garnier.

3,339,484 9/1967 Pannier 101219 EDGAR S. BURR, Primary Examiner US. Cl. X.R. 

